No, you cook them for the only the amount of time you'd cook one chicken. You're not cooking a 15 pound bird, you are cooking 2 7 pound birds, and a 7 pound bird, even sitting next to another 7 pound bird will cook in an hour and a half.
Just make sure you temp them both, especially in the middle of the pan, where less hot air was circulating. They have to temp at 160.